This invention relates to measuring of rotary positions and more particularly, to a low cost approach to measuring high resolution rotary position of a sinusoidally controlled motor.
It is known in the art relating to electric motors that polyphase permanent magnet (PM) brushless motors with a sinusoidal field offer the capability of providing low torque ripple, noise and vibration compared to motors with a trapezoidal field. Theoretically, if a motor controller can produce polyphase sinusoidal currents with the same frequency as that of the sinusoidal back EMFs, the torque output of the motor will be a constant, and zero torque ripple can be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from those assumptions of pure sinusoidal back EMF and current waveforms. The deviations will usually result in parasitic torque ripple components at various frequencies and magnitudes. The methods of torque control can influence the level of this parasitic torque ripple.
One method for torque control of a permanent magnet motor with a sinusoidal back EMF is accomplished by controlling the motor phase currents so that its current vector is aligned with the back EMF. This control method is known as the current mode control method. In such a method, the motor torque is proportional to the magnitude of the current. However, the current mode control method has some drawbacks. The current mode control method requires a complex controller for digital implementation. The controller requires two or more A/D channels to digitize the current feedback from current sensors. In a three phase system, it is convenient to transform the three-phase variables into a two dimensional d-q synchronous frame which is attached to the rotor and design the controller in the d-q frame. But, due to considerable calculations and signal processing involved in performing the d-q transformation, reverse d-q transformation and P-I loop algorithms, a high speed processor such as a digital signal processor (DSP) has to be used to update the controller information every data sampling cycle.
Electric power steering (EPS) has been the subject of development by auto manufacturers and suppliers for over a decade because of its fuel economy and ease-of-control advantages compared with the traditional hydraulic power steering (HPS). However, commercialization of EPS systems has been slow and is presently limited to small and midget-class cars due to cost and performance challenges. Among the most challenging technical issues is the pulsating feel at the steering wheel and the audible noise associated with the type of high performance electric drives needed to meet the steering requirements.
The choice of motor type for an EPS is important, because it determines the characteristics of the drive and the requirements on the power switching devices, controls, and consequently cost. Leading contenders are the Permanent Magnet (PM) brushless motor, the Permanent Magnet (PM) commutator-type motor and the switched reluctance (SR) motor, each of the three options has its own inherent advantages and limitations. The PM brushless motor, was chosen based on years of experimenting with commutator-type motors. The large motor size and rotor inertia of commutator-type motors limit their applicability to very small cars with reduced steering assist requirements. Additionally, the potential for brush breakage that may result in a rotor lock necessitates the use of a clutch to disconnect the motor from the drive shaft in case of brush failure. SR drives offer an attractive, robust and low cost option, but suffer from inherent excessive torque pulsation and audible noise, unless special measures are taken to reduce such effects. For EPS column assist applications, the motor is located within the passenger compartment and therefore must meet stringent packaging and audible noise requirements that the present SR motor technology may not satisfy. Therefore, the PM brushless motor with its superior characteristics of low inertia, high efficiency and torque density, compared to commutator motors, appears to have the potential for not only meeting the present requirements but also of future high performance EPS systems of medium and large vehicles.
Despite the relatively low levels of torque ripple and noise of EPS systems using conventional PM brushless motors, they are no match to the smoothness and quietness of HPS with decades-long history of performance refinement efforts. Consumers are reluctant in compromising such features. Therefore, a new torque ripple free (TRF) system is needed, which as the name indicates would eradicate the sources of torque ripple (under ideal conditions) and reduces the noise level considerably. The near term goal is to enhance the performance of EPS systems with the long term objective of increasing acceptability of EPS systems for broader usage.
Several performance and cost issues have stood in the way of broad-based EPS commercialization regardless of the technology used, but with varying degree of difficulty. This requires that following be addressed:
1. Steering Feel: The key to the wider use of EPS is the ability to reproduce the smoothness feel of hydraulic steering systems at affordable prices. Pulsating torque produced by motors would be felt at the steering wheel, if not reduced to very low levels.
2. Audible Noise: The EPS audible noise is mainly emanating from the motor and gearbox. The gear noise is obviously mechanical due to lash caused by manufacturing tolerances. The motor-caused noise is mainly a result of structural vibration excited by torque pulsation and radial magnetic forces in brushless motors and by the commutator/brush assembly in commutator motors.
In order to better appreciate the elements of the new scheme, a more detailed discussion about the torque ripple and noise generation mechanisms with a focus on PM brushless motors is presented in the following sections.
There are two sources for torque ripple in a conventional PM brushless motors: 1. cogging or detent torque, and 2. commutation torque.
The cogging torque is due to the magnetic interaction between the permanent magnets and the slotted structure of the armature. It exists in both brushless and brush-type machines at all speeds and loads, including no-load. The magnetic attraction force exerted on each individual stator tooth, as the magnet leading edge approaches, produces a positive torque, while the force between the tooth and the trailing edge causes a negative torque. The instantaneous value of the cogging torque varies with rotor position and alternates at a frequency that is proportional to the motor speed and the number of slots. The amplitude of the cogging torque is affected by some design parameters, such as slot opening/slot pitch ratio; magnet strength; and air gap length, while its profile could be altered by varying the pole arc/pole pitch ratio. Careful selection of these parameters can lead to reducing cogging torque, but this approach is limited by practical and performance constraints.
A more common and effective approach is by skewing either the stator teeth or the rotor magnet longitudinally, which provides for a gradual transition as the magnet moves under a stator tooth. Theoretically, a skew amount of one slot pitch should eliminate cogging. However, due to practical factors such as magnetic leakage end effects, skew variation due to tolerances, and eccentricity, some undesirable cogging remains.
In a voltage controlled system, such as a torque ripple free motor with sinusoidal inverter and voltage mode operation, a reduction of torque ripple typically can be achieved by way of utilizing a high resolution sensor. However, the high resolution sensor has drawbacks such as high cost, high maintenance, high error rate, etc. Therefore it is desirable to have a low cost, low maintenance, low error rate, and so forth approach to measuring with acceptable resolution for rotary positions of an electric machine.
One solution to the above mentioned problems is by using a low resolution position sensor in a method to estimate a position of the motor rotor in between sensed position signals. For example, the method can be as simple as using a speed, be it estimated or measured, to estimate the position during the time interval from a previously sensed known position until the next position value is sensed or known. Then, position values are acquired by integration of sensed speed values. Because integration is a very robust and stable operation, one can take the integral of the speed and use the previous measured position as an initial condition. With the initial condition set, it is sufficient to have a good estimate of the position until a next position signal is sensed.
This disclosure teaches a method and apparatus for measuring a rotary position of an electric machine that includes reading a pulse counter that counts a sequence of pulses. The sequence of pulses is related to the rotary position. A determination is then made as to whether a new pulse has been detected and a computation of a new angle that is related to the rotary position is generated. A speed measuring circuit that includes an input having a set of position signals and an output having a set of speed signals may be included in the method for measuring a rotary position of an electric machine.
It is noted that a low resolution position sensor introduces undesirable xe2x80x9cstep-ladderxe2x80x9d noise into what should be a smoothly sinusoidal position signal. Poor position resolution distorts the sinusoidal build up function for the command voltage along with the command amplitude. This invention eliminates these problems while using a low cost, low-resolution position sensor.